Probably the most common reason for poor function after a knee replacement is related to the way the surgeon aligned the knee replacement. The most common way knee replacements are aligned is using MECHANICAL alignment. Most surgeons are trained to place knee replacements this way. Navigation and Robot assistance has attempted to increase the accuracy of MECHANICAL alignment in an effort to improve outcomes ie patient satisfaction however studies have not shown either navigation or robot use to improve outcomes.
Mechanical alignment historically has been promoted with the thought/idea that aligning the implant perpendicular to the ground would decrease the chance of the implant loosening over time. However, research has shown that choosing an alignment called KINEMATIC ALIGNMENT or PATIENT SPECIFIC ALIGNMENT or PERSONALIZED ALIGNMENT that matches the patient’s native anatomy, does not increase the rate of loosening of the implant.
Mechanical alignment requires the surgeon to make a cut of the end of the thigh bone/femur that is perpendicular to the center of the hip. This may or may not be the same angle as the patients native bone/joint angle. Poor function occurs when the mechanical angle differs significantly than the native/anatomic/patient specific bone/joint angle. All of our femur bone angles are aligned differently at the end of the bone.
As an example – if a surgeon puts the distal femur bone/joint angle at 5 degrees but the patient’s native angle is 9 degrees, this means that the amount of bone removed from each side of the knee is different – more bone is removed from the medial/inner aspect of the bone and less is removed from the outer/lateral side of the knee. This means that there is less space in the knee to allow for movement/range of motion on the outer/lateral side of the knee and the inner side may be loose/lax. Each degree equates to about 1 mm of space. So this would mean the lateral knee is ‘overstuffed’ by 4mm leading to stiffness and loss/lack of range of motion.
The tibial alignment is pretty much equal – mechanical vs anatomic, differing by 2mm at the end of the tibia which is negligible/equivalent, so any changes to the femur directly affect the joint balance and function.
This alignment strategy is exacerbated by our MECHANICAL alignment training to place the new femoral implant/joint angle 3 degrees rotated from our native/anatomic posterior condylar axis. So more bone is removed from the medial posterior condyle and less from the lateral posterior condyle. This overstuffs the lateral posterior space by 3mm. This is why the average registry data worldwide has a flexion angle equal to 115 degrees, far less than normal – surgeons have been trained to “overstuff” the knee in an attempt to help patella/knee cap tracking. If a patient does not have a dislocating kneecap/poor tracking due to their native anatomy, one would expect the kneecap to track normally with their native alignment/rotation.
So if you change a persons flexion arch to a different/non-anatomic arch, it may represent why a patient is unhappy with the TKA outcome. I think this is one reason why 15% of TKA patients are unhappy with their TKA.
This also happens if the shin bone/tibia is placed in less slope than the native knee. so if someone has a 5 degree slope backwards on the tibia/shin bone, and the knee is placed at 3 degrees of posterior slope, this equates to 2 degrees or 2 mm of ‘overstuffing’ the back of the knee leading to less bend or flexion.
There are ways that surgeons increase space to allow for the overstuffing of the lateral and posterior knee — releasing of ligaments. This leads to more pain post operatively and is often incomplete. The knee has circumferential attachments between the tibia and femur and only releasing the main ligaments on the sides unfortunately is incomplete. The idea that only 2 main ligaments create the bend direction or radius of bend/flexion refers to a ‘single radius’ idea of knee replacement. This idea however suggests that the other points of attachment are insignificant and oversimplifies the bend and rotation that occurs with the native knee. With a ‘single radius’ interpretation of knee function, a person ignores the native tibial internal rotation that occurs with flexion.
Another major reason that knee replacements fail is due to an inflammatory reaction to the implant. Some patients refer to this as ‘rejection’ of the implant. This is actually an allergy to the implant metal polish. The metal polish of the majority of knee replacements is CoCr (cobalt chrome), which contains nickel. Those 3 metals are the most common metals that humans are allergic. Some patients can develop an allergy to metal after its implanted.
The common presentation of a metal allergy reaction to a knee replacement is swelling, pain, poor flexion/extension due to the body creating scar tissue, ‘a rind’ of tissue, around the implant. Until a surgeon has seen this response and confirmed the allergy by testing, they are unlikely to ‘believe’ that humans can react to knee implants. It is rare, but real and should be evaluated with allergy testing if there is poor function following knee arthroplasty.
Beckers G, Meneghini RM, Hirschmann MT, Kostretzis L, Kiss MO, Vendittoli PA. Ten Flaws of Systematic Mechanical Alignment Total Knee Arthroplasty. J Arthroplasty. 2024 Mar;39(3):591-599. doi: 10.1016/j.arth.2023.11.023. Epub 2023 Nov 23. PMID: 38007204.